Inter-laboratory Testing – Honey

Food analysis using Isotopic Techniques – Proficiency Testing Scheme.
Results of carbon-13 and nitrogen-15 analysis of Honey undertaken in the first round of 2008 are shown below. The Z-scores (see explanation below the charts) show that the participating laboratories were in good agreement with each other and our own Z scores are comfortably in the desired range.

The evaluation of data is performed according to the ISO/IUPAC/AOAC International Harmonised Protocol for Proficiency Testing of analytical laboratories. Individual laboratories results are expressed as Z-scores:

Z = (x-X)/s

Where :
x is the reported result from the participating laboratory.
X is the assigned or “true” value for the analyte being determined. In this exercise the assigned values are taken as the robust mean of all reported results.
s is the target value for standard deviation. In this exercise the target SD values are derived from recently reported collaborative trial results or from the robust standard deviation of all reported values.

If X and s are good estimates of the population mean and standard deviation, and the underlying distributions are normal, Z is normally distributed with a mean of zero and unit standard deviation.

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